1. Field of the Invention
The present invention relates to systems and methods for broadcasting information signals, and in particular to a system and method for reducing interference between independent terrestrially-based and space-based broadcasting systems.
2. Description of the Related Art
It has been proposed to cooperatively share the current Broadcasting-Satellite Service (BSS) frequency bands to allow additional programming material to be transmitted to subscribers of other broadcast systems using the same frequency bands as currently used by BSS satellites. The other systems may be implemented through the use of non-geostationary orbit (GSO) and/or terrestrially-based transmitters to transmit the additional programming. Such systems typically rely on spatial diversity to minimize the probability of interference. This usually requires a BSS satellite ground antenna having highly directional, monocular sensitivity characteristics in order to realize low interference levels.
Unfortunately, existing BSS antennae do not exhibit a highly directional sensitivity characteristic. Instead, as described in application Ser. No. 09/480,089, entitled “METHOD AND APPARATUS FOR MITIGATING INTERFERENCE FROM TERRESTRIAL BROADCASTS SHARING THE SAME CHANNEL WITH SATELLITE BROADCASTS USING AN ANTENNA WITH POSTERIOR SIDELOBES,” which application is hereby incorporated by reference, existing BSS antennae exhibit a sensitivity characteristic that includes substantial sensitivity in a rearward direction. They also exhibit sensitivity characteristics in the sideward and upward directions. This sensitivity can result in substantial interference between transmissions from BSS satellites and transmissions from non-GSO or terrestrial sources.
Solutions have been proposed to reduce interference, including those described in the related applications described above. However, even when using the methods described in the above-referenced patent applications, excessive interference with existing BSS system broadcasts may result.
Two-way communication systems has been proposed which would allow consumers to transmit narrowband interstitial return path signals within the 12.2–12.7 GHz frequency bands already used by digital broadcast service providers. However, these systems limit the interstitial transmission to reverse link transmissions (from subscribers to terrestrially based antennae). One of the reasons that such systems have been limited in the past to reverse link transmissions is because it has been assumed that the limited bandwidth available on the interstitial signals is suitable only for low data rate transmissions.
Interstitial return link transmissions are typically made possible by the use of high-directivity ground antennae at the subscriber's location. Such antennae make it feasible for each subscriber antenna to direct energy in a narrow beam to the terrestrial antenna, thereby spatial diversity can be used to minimize transmission interference. However, it is not economically feasible for the terrestrial antenna to direct an individual beam to each subscriber. Hence, such methods are generally inapplicable to forward link communications.
What is needed is a further method for isolating current BSS transmissions from proposed forward link transmissions from terrestrially based transmitters. The present invention satisfies that need at least in part by taking advantage of the spatial diversity provided by multiple and spot-beam capable broadcast satellites, and the use of spatial and polarization diversity to minimize interference while maximizing forward path transmission bandwidth.